cc: Stop using magic numbers to detect if we can use partial updates.

cc: Stop using magic numbers to detect if we can use partial updates.

We were using size_t's max value to detect if we can always use
partial updates, but this is pretty magical. Instead, enable it when
1) The renderer allows partial updates at all (not ubercomp), and
2) Using single thread proxy.

R=reveman@chromium.org
BUG=234233
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=226924

[reveman, 2013-10-03 19:34:47]

lgtm

[commit-bot: I haz the power, 2013-10-03 19:39:37]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/danakj@chromium.org/25912003/1

[commit-bot: I haz the power, 2013-10-04 01:27:53]

Change committed as 226924